Downlink multiple input multiple output enhancements for single-cell with remote radio heads

ABSTRACT

A base station selects a subset of at least one geographically separated antennas for each of the plurality of user equipments. The base station forms at least layer of data stream including modulated symbols, precodes the data stream via multiplication with the NT-by-N precoding matrix where N is the number of said layers and NT is the number of transmit antenna elements and transmits the precoded layers of data stream to the user equipment via the selected geographically separated antennas. The base station signals the subset of the plurality of geographically separated antennas via higher layer Radio Resource Control or via a down link grant mechanism. The base station optionally does not signal the subset of the plurality of geographically separated antennas to the corresponding mobile user equipment.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/955,513 filed Apr. 17, 2018, which is a continuation of U.S. patentapplication Ser. No. 14/613,176 filed Feb. 3, 2015, now U.S. Pat. No.10,033,447 issued Jul. 24, 2018. This application is also a continuationof U.S. patent application Ser. No. 15/955,501 filed Apr. 17, 2018,which is a continuation of U.S. patent application Ser. No. 14/613,176filed Feb. 3, 2015, now U.S. Pat. No. 10,033,447 issued Jul. 24, 2018.U.S. patent application Ser. No. 14/613,176 is a continuation of U.S.patent application Ser. No. 13/451,718 filed Apr. 20, 2012, now U.S.Pat. No. 8,948,293 issued Feb. 3, 2015, which claims the benefit of U.S.Provisional Application No. 61/477,341 filed Apr. 20, 2011.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is wireless communication such aswireless telephony.

BACKGROUND OF THE INVENTION

With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbolsare transmitted on multiple carriers that are spaced apart to provideorthogonality. An OFDM modulator typically takes data symbols into aserial-to-parallel converter, and the output of the serial-to-parallelconverter is considered as frequency domain data symbols. The frequencydomain tones at either edge of the band may be set to zero and arecalled guard tones. These guard tones allow the OFDM signal to fit intoan appropriate spectral mask. Some of the frequency domain tones are setto values which will be known at the receiver. Among these areCell-specific Channel State Information Reference Signals (CSI-RS) andDedicated or Demodulating Reference Signals (DMRS). These referencesignals are useful for channel estimation at the receiver. In amulti-input multi-output (MIMO) communication systems with multipletransmit/receive antennas, the data transmission is performed viaprecoding. Here, precoding refers to a linear (matrix) transformation ofa L-stream data into P-stream where L denotes the number of layers (alsotermed the transmission rank) and P denotes the number of transmitantennas. With the use of dedicated (user-specific) DMRS, a transmitter(base station, also termed an eNodeB or eNB) can perform any precodingoperation which is transparent to a user equipment (UE) which acts as areceiver. At the same time, it is beneficial for the base station toobtain a recommendation on the choice of precoding matrix from the userequipment. This is particularly the case for frequency-divisionduplexing (FDD) where the uplink and downlink channels occupy differentparts of the frequency bands, i.e. the uplink and downlink are notreciprocal. Hence, a codebook-based feedback from the UE to the eNodeBis preferred. To enable a codebook-based feedback, a precoding codebookneeds to be designed.

To extend cell coverage and service over a wide area, employing remoteradio heads (RRHs) is beneficial. Multiple units of RRH are distributedover a wide area and act as multiple distributed antennas for theeNodeB. For downlink transmissions, each RRH unit is associated with aunit of transmit radio device—which constitutes to at least one antennaelement along with the associated radio and analog front-end devices.Each unit of RRH is positioned relatively far from the eNodeB andtypically connected via a low-latency line such as fiber optic link.Some exemplary configurations are depicted in FIG. 1 where six RRHs areutilized. Depending on whether each RRH is equipped with a single ordual antenna elements, up to 12 antenna elements can be supported.

While the LTE cellular standard along with its further evolutionLTE-Advanced (also known as the E-UTRA and further enhanced E-UTRA,respectively) offer a solid support of MIMO technology, the MIMOmechanism supported in the specification was primarily designed forco-located antenna elements. In Rel-10 LTE-A, some support for RRH-basedconfiguration was provisioned for the use in the context of thecoordinated multi-point (COMP) transmission. While the precedingapproaches provide improvements in wireless communications, the presentinventors recognize that still further improvements in downlink (DL)spectral efficiency are possible when RRH-based configuration isemployed. Accordingly, the preferred embodiments described below aredirected toward these problems as well as improving upon the prior art.

SUMMARY OF THE INVENTION

A method and apparatus of wireless communication between a base stationhaving a plurality of geographically separated antennas and a pluralityof user equipments. The base station selects a subset of at least one ofthe geographically separated antennas for each of the plurality of userequipments. The base station forms at least one layer of data streamincluding modulated symbols for each user equipment. The base stationprecodes the data stream for each user equipment via multiplication withthe NT-by-N precoding matrix where N is the number of said layers and NTis the number of transmit antenna elements. The base station transmitsthe precoded layers of data stream to the user equipment via theselected geographically separated antennas.

The base station signals the subset of the plurality of geographicallyseparated antennas to the corresponding user equipment via higher layerRadio Resource Control or via a down link grant mechanism. The basestation optionally does not signal the subset of the plurality ofgeographically separated antennas to the corresponding mobile userequipment.

The base station semi-statically selects the subset of the plurality ofgeographically separated antennas for user equipment having a Dopplerindicating a rate of motion less than a predetermined amount. The basestation dynamically selects the subset of the plurality ofgeographically separated antennas for user equipment having a Dopplerindicating a rate of motion greater than the predetermined amount. Inthis case the base station selection may be responsive to a userequipment recommendation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 illustrates an exemplary prior art wireless communication systemto which this application is applicable;

FIG. 2 shows a number of possibilities in how RRHs are deployed within asingle cell according to the prior art;

FIG. 3 is a flow chart of the operation in this invention;

FIG. 4 is a flow chart illustrating an embodiment of one processingblock of FIG. 3 ; and

FIG. 5 is a block diagram illustrating internal details of a basestation and a mobile user equipment in the network system of FIG. 1suitable for implementing this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an exemplary wireless telecommunications network 100. Theillustrative telecommunications network includes base stations 101, 102and 103, though in operation, a telecommunications network necessarilyincludes many more base stations. Each of base stations 101, 102 and 103(eNB) are operable over corresponding coverage areas 104, 105 and 106.Each base station's coverage area is further divided into cells. In theillustrated network, each base station's coverage area is divided intothree cells. Handset or other user equipment (UE) 109 is shown in Cell A108. Cell A 108 is within coverage area 104 of base station 101. Basestation 101 transmits to and receives transmissions from UE 109. As UE109 moves out of Cell A 108 and into Cell B 107, UE 109 may be handedover to base station 102. Because UE 109 is synchronized with basestation 101, UE 109 can employ non-synchronized random access toinitiate handover to base station 102.

Non-synchronized UE 109 also employs non-synchronous random access torequest allocation of up-link 111 time or frequency or code resources.If UE 109 has data ready for transmission, which may be traffic data,measurements report, tracking area update, UE 109 can transmit a randomaccess signal on up-link 111. The random access signal notifies basestation 101 that UE 109 requires up-link resources to transmit the UEsdata. Base station 101 responds by transmitting to UE 109 via down-link110, a message containing the parameters of the resources allocated forUE 109 up-link transmission along with a possible timing errorcorrection. After receiving the resource allocation and a possibletiming advance message transmitted on down-link 110 by base station 101,UE 109 optionally adjusts its transmit timing and transmits the data onup-link 111 employing the allotted resources during the prescribed timeinterval.

Base station 101 configures UE 109 for periodic uplink soundingreference signal (SRS) transmission. Base station 101 estimates uplinkchannel quality information (CSI) from the SRS transmission.

The preferred embodiments of the present invention provide improvedcommunication through multi-antenna transmission over multiple units ofremote radio heads (RRHs). There are a number of possibilities in howRRHs are deployed within a single cell.

It is useful to first identify and summarize the characteristics ofsingle-cell deployment with RRHs. In principle, there are a number ofpossibilities in how RRHs are deployed within a single cell. Somepossibilities are illustrated in FIG. 2 . Each RRH unit may be: asingle-polarized (dipole) antenna element; a dual-polarized antennaelement; a small (e.g. 2-element) antenna array where each element iseither single or dual-polarized.

FIG. 2 illustrates one cell of a wireless communication system withplural geographically separated antenna. Each cell 210, 220 and 230 have6 RRH units. In cell 210 each antenna is one single-polarized antenna211. In cell 220 each antenna is one dual-polarized antenna 221. In cell230 each antenna is two single-polarized antennas 231. A RRH unit refersto a geographically separated unit which may include single or multipleantenna element(s) and/or RF unit(s). For DL MIMO some consideration ofapplicable configurations is beneficial. Different configurations mayimpose different design constraints.

Multiple RRHs within a single cell can be regarded as a distributed MIMOsystem where different RRH units undergo different delays from/to agiven UE. There is significant gain imbalance relative to the UE acrossdifferent RRH units. The associated spatial channels tend to be almostuncorrelated across different RRH units. These characteristics imposesome design constraints if some potential enhancements are to beincluded solely for this scenario.

Due to the characteristics mentioned above a single-cell with multipleRRHs can be operated as follows. For a given UE, the eNB chooses asubset of all the available RRH units. This solution is technicallysound from capacity perspective. While a subset may contain all theavailable RRHs within the cell, it is unnecessary when the cell issufficiently large and coverage improvement takes more precedence overcapacity improvement. Consequently, this RRH subset of all the availableRRH units is UE specific.

Dynamic RRH subset selection is expected to be better but costly insignaling requirements. A new DL grant mechanism or a System InformationBlock Broadcast (SIB-x) on a dedicated Broadcast CHannel (BCH) with0≤x≤13 which signals the RRH subset is needed. Dynamic RRH subsetselection also allows the possibility for the UE to recommend the RRHsubset. This leads to a new CSI feedback mechanism. The UE needs toperform measurements on all the available RRHs.

Semi-static RRH subset selection is simpler. A Rel. 8 mechanism whichindicates the number of antenna ports, which is a broadcast parameterfor Rel. 8, can be used. However this needs to be UE specific. To signalthe RRH subset, some additional Radio Resource Control (RRC) signalingcapability is needed.

In a first alternative the UE may not need to know which RRH subset isused particularly if the CSI-RS is UE specific. That is, if the RRHsubset is transparent to all the UEs.

In a second alternative all the UEs may know all the RRHs. Thus the UEneeds to know the RRH subset. In this alternative the RRH subset is RRCsignaled. This may lead to some further complication and thus the firstalternate above may be preferred.

This invention includes a combination (hybrid) of dynamic andsemi-static signaling. The semi-static signaling configures asemi-static subset of RRHs via higher-layer RRC signaling. Thissemi-static signaling includes a list of CSI-RS patterns. Thus the UEknows the association between each pattern and the corresponding RRH.The semi-static signaling further specifies the relationship betweeneach RRH and the set of antenna ports (7, 8, . . . 6+v, where v is thenumber of layers) on which the UE receives its UE-RS. This ensures thatwhen the subset of RRHs the UE uses to communicate changes, the UE knowsthe corresponding CSI-RS and UE-RS patterns for estimating its channelsto the new subset of RRHs to which it communicates. Dynamic signaling isused to select a smaller subset from the semi-static subset. In a firstexample this uses a DL grant mechanism. The DL grant carries anadditional field which informs the UE of the assigned subset of RRHs orRRH units. In a second example the dynamical signaling is conveyed viadedicated signaling on SIB-x with 0≤x≤13.

Because the RRH units are well distributed across the cell, semi-staticsignaling of the RRH subset is expected to be sufficient in mostscenarios. This applicable when the cell is large and/or the UE moves ata reasonable speed. This would probably not be applicable for UE on ahigh-speed train where RRHs are deployed along a subway tunnel toprovide reasonable coverage for a UE inside the subway. In this case,the UE moves at a very high speed of about 350 kilometers per hour andthe RRH subset may change rapidly. The hybrid scheme is beneficial inthis case because it allows faster update of the RRH subset. The eNbdetects the speed of motion of a UE through the size of the Dopplershift in its Up Link transmissions.

Because the RRH subset is UE specific, the CSI-RS configuration alsoneeds to be UE specific. A UE specific CSI-RS configuration is supportedin Rel. 10. Thus the number of antenna ports, the CSI-RS pattern, themuting pattern if muting is configured can be made UE specific. The Rel.10 UE specific CSI-RS support seems sufficient especially for the abovefirst alternative for semi-static scheduling. If second semi-staticscheduling alternative is used, the RRH subset which corresponds to thesubset of all the available CSI-RS ports can be mapped directly onto theRRH units.

FIG. 3 is a flow chart of the operation of the eNB in this invention. Indecision step 301 the eNB determines whether the RRH subset for aparticular UE needs to be updated. If this is true (Yes at decisionblock 301), then in processing block 302 the eNB selects the new RRHsubset for the UE. Thereafter the eNB communicates with the UE using theselected subset in processing block 303. If this is not true (No atdecision block 301), the eNB communicates with the UE using the selectedsubset in processing block 303. In this event the selected RRH subset isthe prior RRH subset. Because the RRH subset of this invention is UEspecific, the eNB needs to perform this process for each UE.

FIG. 4 is a flow chart illustrating an embodiment of processing block302 of FIG. 3 . Some of the operations in FIG. 4 are preformed by theeNB and some are performed by the UE. Processing block 302 of thisembodiment begins at start block 401. Decision block 402 determineswhether the Doppler of the particular UE is greater than a predeterminedlimit. This process is preformed by the eNB. If the Doppler is notgreater than the limit (No at decision block 402), then the eNBsemi-statically selects the new RRH subset in processing block 403. Indecision block 404 the eNB determines whether the UE is to be signaledof the RRH subset. The above description noted that signaling the UE ofthe selected RRH subset is optional. If the UE is to be signaled of theRRH subset (Yes at decision block 404), then the eNB signals the UE ofthe selected RRH subset in processing block 405. The above descriptionstates that this notification occurs via higher-layer RRC signaling whensemi-static selection is used. Processing block 302 is exited toprocessing block 303 via continue block 406.

If the UE is not to be signaled of the RRH subset (No at decision block404), then the eNB determines at decision block 407 whether the CSI-RSis UE specific. If this is not true (No at decision block 407), thenprocessing block 302 is exited to processing block 303 via continueblock 406. If this is true (Yes at decision block 407), then eNB selectsa RRH subset that corresponds to the UE specific CSI-RS. The particularUE knows of this correlation between UE specific CSI-RS and UE specificRRH subset and communicates with the eNB accordingly. Processing block302 is exited to processing block 303 via continue block 406.

If the Doppler is greater than the limit (Yes at decision block 402),then the eNB is optionally responsive to RRH subset recommendation fromthe UE. The eNB then dynamically selects the RRH subset for theparticular UE in processing block 410. The eNB signals the UE of theparticular RRH subset in processing block 411. As noted above thissignaling can be via a DL grant carrying an additional field of viadedicated signaling on SIB-x. Processing block 302 is exited toprocessing block 303 via continue block 406.

FIG. 5 is a block diagram illustrating internal details of an eNB 1002and a mobile UE 1001 in the network system of FIG. 1 . Mobile UE 1001may represent any of a variety of devices such as a server, a desktopcomputer, a laptop computer, a cellular phone, a Personal DigitalAssistant (PDA), a smart phone or other electronic devices. In someembodiments, the electronic mobile UE 1001 communicates with eNB 1002based on a LTE or Evolved Universal Terrestrial Radio Access Network(E-UTRAN) protocol. Alternatively, another communication protocol nowknown or later developed can be used.

Mobile UE 1001 comprises a processor 1010 coupled to a memory 1012 and atransceiver 1020. The memory 1012 stores (software) applications 1014for execution by the processor 1010. The applications could comprise anyknown or future application useful for individuals or organizations.These applications could be categorized as operating systems (OS),device drivers, databases, multimedia tools, presentation tools,Internet browsers, emailers, Voice-Over-Internet Protocol (VOIP) tools,file browsers, firewalls, instant messaging, finance tools, games, wordprocessors or other categories. Regardless of the exact nature of theapplications, at least some of the applications may direct the mobile UE1001 to transmit UL signals to eNB (base-station) 1002 periodically orcontinuously via the transceiver 1020. In at least some embodiments, themobile UE 1001 identifies a Quality of Service (QoS) requirement whenrequesting an uplink resource from eNB 1002. In some cases, the QoSrequirement may be implicitly derived by eNB 1002 from the type oftraffic supported by the mobile UE 1001. As an example, VOIP and gamingapplications often involve low-latency uplink (UL) transmissions whileHigh Throughput (HTP)/Hypertext Transmission Protocol (HTTP) traffic caninvolve high-latency uplink transmissions.

Transceiver 1020 includes uplink logic which may be implemented byexecution of instructions that control the operation of the transceiver.Some of these instructions may be stored in memory 1012 and executedwhen needed by processor 1010. As would be understood by one of skill inthe art, the components of the uplink logic may involve the physical(PHY) layer and/or the Media Access Control (MAC) layer of thetransceiver 1020. Transceiver 1020 includes one or more receivers 1022and one or more transmitters 1024.

Processor 1010 may send or receive data to various input/output devices1026. A subscriber identity module (SIM) card stores and retrievesinformation used for making calls via the cellular system. A Bluetoothbaseband unit may be provided for wireless connection to a microphoneand headset for sending and receiving voice data. Processor 1010 maysend information to a display unit for interaction with a user of mobileUE 1001 during a call process. The display may also display picturesreceived from the network, from a local camera, or from other sourcessuch as a Universal Serial Bus (USB) connector. Processor 1010 may alsosend a video stream to the display that is received from various sourcessuch as the cellular network via RF transceiver 1020 or the camera.

During transmission and reception of voice data or other applicationdata, transmitter 1024 may be or become non-synchronized with itsserving eNB. In this case, it sends a random access signal. As part ofthis procedure, it determines a preferred size for the next datatransmission, referred to as a message, by using a power threshold valueprovided by the serving eNB, as described in more detail above. In thisembodiment, the message preferred size determination is embodied byexecuting instructions stored in memory 1012 by processor 1010. In otherembodiments, the message size determination may be embodied by aseparate processor/memory unit, by a hardwired state machine, or byother types of control logic, for example.

eNB 1002 comprises a Processor 1030 coupled to a memory 1032, symbolprocessing circuitry 1038, and a transceiver 1040 via backplane bus1036. The memory stores applications 1034 for execution by processor1030. The applications could comprise any known or future applicationuseful for managing wireless communications. At least some of theapplications 1034 may direct eNB 1002 to manage transmissions to or frommobile UE 1001.

Transceiver 1040 comprises an uplink Resource Manager, which enables eNB1002 to selectively allocate uplink Physical Uplink Shared CHannel(PUSCH) resources to mobile UE 1001. As would be understood by one ofskill in the art, the components of the uplink resource manager mayinvolve the physical (PHY) layer and/or the Media Access Control (MAC)layer of the transceiver 1040. Transceiver 1040 includes at least onereceiver 1042 for receiving transmissions from various UEs within rangeof eNB 1002 and at least one transmitter 1044 for transmitting data andcontrol information to the various UEs within range of eNB 1002.

The uplink resource manager executes instructions that control theoperation of transceiver 1040. Some of these instructions may be locatedin memory 1032 and executed when needed on processor 1030. The resourcemanager controls the transmission resources allocated to each UE 1001served by eNB 1002 and broadcasts control information via the PDCCH.

Symbol processing circuitry 1038 performs demodulation using knowntechniques. Random access signals are demodulated in symbol processingcircuitry 1038.

During transmission and reception of voice data or other applicationdata, receiver 1042 may receive a random access signal from a UE 1001.The random access signal is encoded to request a message size that ispreferred by UE 1001. UE 1001 determines the preferred message size byusing a message threshold provided by eNB 1002. In this embodiment, themessage threshold calculation is embodied by executing instructionsstored in memory 1032 by processor 1030. In other embodiments, thethreshold calculation may be embodied by a separate processor/memoryunit, by a hardwired state machine, or by other types of control logic,for example. Alternatively, in some networks the message threshold is afixed value that may be stored in memory 1032, for example. In responseto receiving the message size request, eNB 1002 schedules an appropriateset of resources and notifies UE 1001 with a resource grant.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling with the inventive scope as defined by the followingclaims. Other combinations will be readily apparent to one of ordinaryskill in the art having access to the instant specification.

What is claimed is:
 1. A method of wireless communication between a basestation having a plurality of geographically separated antennas and aplurality of user equipments, comprising the steps of: selecting at thebase station a subset of at least one of the geographically separatedantennas for each of the plurality of user equipments based on a Dopplerof the respective user equipment; forming at least one layer of datastream for each user equipment, each of the data streams includingmodulated symbols; precoding the at least one layer of data stream foreach user equipment via multiplication with the NT-by-N precoding matrixwhere N is the number of said layers and NT is the number of transmitantenna elements; and transmitting the precoded layers of data stream toeach user equipment via the selected geographically separated antennas.2. The method of claim 1, wherein: said step of selecting includes thebase station signaling said subset of the plurality of geographicallyseparated antennas to the corresponding user equipment via higher layerRadio Resource Control.
 3. The method of claim 1, wherein: said step ofselecting includes the base station signaling said subset of theplurality of geographically separated antennas to the corresponding userequipment via a down link grant mechanism.
 4. The method of claim 1,further comprising: not signaling said subset of the plurality ofgeographically separated antennas to the corresponding mobile userequipment from the base station to the mobile user equipment.
 5. Themethod of claim 1, wherein: said step of selecting occurringsemi-statically for user equipment having a Doppler indicating a rate ofmotion less than a predetermined amount; and said step of selectingoccurring dynamically for user equipment having a Doppler indicating arate of motion greater than said predetermined amount.
 6. The method ofclaim 5, wherein: said step of selecting dynamically includes the userequipment recommending a RRH subset recommendation to the base station,and the base station responsive to the user equipment recommendation. 7.A wireless communication system comprising: a plurality of userequipment; a base station having a plurality of geographically separatedantennas, said base station operable to select a subset of at least oneof the geographically separated antennas for each of said plurality ofuser equipments based on a Doppler of the respective user equipment,form at least one layer of data stream for each user equipment, each ofthe data streams including modulated symbols, precode the at least onelayer of data stream for each user equipment via multiplication with theNT-by-N precoding matrix where N is the number of said layers and NT isthe number of transmit antenna elements, and transmit the precodedlayers of data stream to each user equipment via the selectedgeographically separated antennas; and wherein each of said plurality ofuser equipment is operable to receive the transmitted the precodedlayers of data stream.
 8. The system of claim 7, wherein: said basestation is further operable to signal said subset of the plurality ofgeographically separated antennas to the corresponding user equipmentvia higher layer Radio Resource Control; and each of the plurality ofuser equipment is operable to receive said signal of said subset of theplurality of geographically separated antennas.
 9. The system of claim7, wherein: said base station is further operable to signal said subsetof the plurality of geographically separated antennas to thecorresponding user equipment via a down link grant mechanism; and eachof the plurality of user equipment is operable to receive said signal ofsaid subset of the plurality of geographically separated antennas. 10.The system of claim 7, wherein: said base station is further operable tosemi-statically select a subset of at least one of the geographicallyseparated antennas for user equipment having a Doppler indicating a rateof motion less than a predetermined amount, and dynamically select asubset of at least one of the geographically separated antennas for userequipment having a Doppler indicating a rate of motion greater than saidpredetermined amount.
 11. The system of claim 10, wherein: said userequipment operable to recommending a RRH subset recommendation to saidbase station; and said base station operable to dynamically select asubset of at least one of the geographically separated antennas for userequipment responsive to said user equipment recommendation.